The present invention generally relates to charge coupled devices (CCDs) and, more particularly, to input circuits for charge coupled devices.
The application of infrared detectors to imaging is growing steadily. As advanced infrared systems are being developed, the requirement for greater sensitivity and resolution is desired for many applications. System performance parameters relating to these requirements always depend on the signal to noise ratio of the many individual detectors comprising the focal plane. The use of multielement focal planes of small closely packed detectors, made possible by the use of charge coupled devices, has helped reduce the noise in such systems, while allowing use of a large number of detectors, and, therefore, better satisfy the sensitivity and resolution requirements.
The coupling circuit between the detectors and the CCDs must transform the signal information from the detectors into charges in the CCD shift registers with minimum degradation of the signal-to-noise ratio of the system. The input circuit must tailor the signal so that the CCD charge capacity is not exceeded when the signal, including the background signal, varies over the system dynamic range. In addition, the CCD must be protected from excess charge from signals beyond the system dynamic range, because excess charge injected into the CCD from one detector input will mix with charge from other inputs, thereby causing so-called image blooming and loss of information.
The gain of such input circuits must be uniform so that every CCD channel has the desired maximum signal capability without saturating the CCD. Some focal plane applications require the capability of handling a very large dynamic range, and, therefore, a very large range of signal or input current to the CCDs. For example, in some applications, the background seen by the detector varies over a range of up to 10,000 to 1. In addition, the input circuit must handle this large range of inputs while maintaining a low noise figure. An ac detector/CCD coupling approach has previously been used in an attempt to handle this dynamic range, however, it has been found that this approach is strongly susceptible to CCD input nonuniformities and CCD input low frequency noise. Further, such approach has no radiometric capability. A technique for handling moderate dynamic range, sometimes referred to as the Charge Equilibrium (CE) circuit, provides charge buffering by use of charge reduction or low gain. This technique reduces noise by use of a standard charge splitting circuit. For further information on these techniques, see the articles entitled "IR/CCD Hybrid Focal Planes" by R. Broudy et al, published in The Proceedings of the Society of Photo-optical Instrumentation Engineers (SPIE), Volume 132, 1978, and "Multiplexed Intrinsic Detector Arrays With Signal Processing (MIDASP)" by M. Gurnee et al, published in The Proceedings of the Society of Photo-optical Instrumentation Engineers (SPIE), Volume 217, 1980.
It is, accordingly, a primary object of the present invention to provide an input circuit for charge coupled devices which circuit may be used in focal plane applications having a large dynamic range.